Add stat to PassiveStream class for last received seqno

Change-Id: I88f0756f6857a9762c6bdbc46884d9dee4af567a
Reviewed-on: http://review.couchbase.org/38201
Reviewed-by: abhinav dangeti <abhinav@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>